Role of pH on Nanostructured SERS Active Substrates for Detection of Organic Dyes.

Surface Enhanced Raman Spectroscopy is commonly used as analytical improvement to conventional Raman spectroscopy, able to respond to qualitative diagnostic enquiries, which involve low-concentrated molecular species in complex matrix. In this paper, we described fabrication, characterization and testing of a type of SERS-active substrates realized specifically to detect pigments in work of art. In particular, we detailed the SERS activity of nanostructured noble metal films deposited by pulsed laser ablation onto glass and polishing sheets substrates. The SERS response of the substrates was tested against the presence of some organic dyes in aqueous solutions. Measurements were performed at different pH values, in acidic or basic range, in order to investigate its role in the adsorption mechanism, thus fostering the SERS amplification. In addition, we checked the possible deterioration of the structural properties of the substrates that could occur in presence of alkaline or acidic environment. SERS activity of the substrates was tested against a commonly dye used as a SERS standard (Blue Methylene). Thereafter, substrates have been tested on two organic dyes (Alizarine red-S and Brazilwood), which had proven to be Raman active but present also either a weak Raman scattering cross section and/or a high fluorescence emission. The substrates have proven effective in amplifying Raman scattering of all dyes, quenching troubling fluorescence effects. Furthermore, they have proven to be stable in the pH range between 3 and 11. Furthermore, we carry out of vibrational DFT-calculation of dyes that provide a complete description of the observed SERS spectra.

Electric-Field-Induced Effects on the Dipole Moment and Vibrational Modes of the Centrosymmetric Indigo Molecule.

Intense static electric fields can strongly perturb chemical bonds and induce frequency shifts of the molecular vibrations in the so-called vibrational Stark effect. Based on a density functional theory (DFT) approach, here, we report a detailed investigation of the influence of oriented external electric fields (OEEFs) on the dipole moment and infrared (IR) spectrum of the nonpolar centrosymmetric indigo molecule. When an OEEF as intense as ∼0.1 V Å-1 is applied, several modifications in the IR spectrum are observed. Besides the notable frequency shift of some modes, we observe the onset of new bands-forbidden by the selection rules in the zero-field case. Such a neat field-induced modification of the vibrational selection rules, and the subsequent variations of the peaks' intensities in the IR spectrum, paves the way toward the design of smart tools employing centrosymmetric molecules as proxies for mapping local electric fields. In fact, here, we show that the ratio between the IR and the Raman intensities of selected modes is proportional to the square of the local field. This indicator can be used to quantitatively measure local fields, not only in condensed matter systems under standard conditions but also in field-emitting-tip apparatus.

Arsenic-nucleotides interactions: an experimental and computational investigation.

Albeit arsenic As(iii) is a well-known carcinogenic contaminant, the modalities by which it interacts with living organisms are still elusive. Details pertaining to the binding properties of As(iii) by common nucleotides such as AMP, ADP and ATP are indeed mostly unknown. Here we present an investigation, conducted via experimental and quantum-based computational approaches, on the stability of the complexes formed by arsenic with those nucleotides. By means of potentiometric and calorimetric measurements, the relative stability of AMP, ADP and ATP has been evaluated as a function of the pH. It turns out that ATP forms more stable structures with As(iii) than ADP which, in turn, better chelates arsenic than AMP. Such a stability sequestration capability of arsenic (ATP > ADP > AMP) has been interpreted on a twofold basis via state-of-the-art ab initio molecular dynamics (AIMD) and metadynamics (MetD) simulations performed on aqueous solutions of As(iii) chelated by AMP and ATP. In fact, we demonstrate that ATP offers a larger number of effective binding sites than AMP, thus indicating a higher statistical probability for chelating arsenic. Moreover, an evaluation of the free energy associated with the interactions that As(iii) establishes with the nucleotide atoms responsible for the binding quantitatively proves the greater effectiveness of ATP as a chelating agent.

Interaction between As(III) and Simple Thioacids in Water: An Experimental and ab Initio Molecular Dynamics Investigation.

Albeit arsenic compounds are ubiquitous in aqueous solutions, the speciation of such a pollutant in natural water mainly depends on its binding capabilities with specific molecules. The features of most of the interactions of arsenic complexes can be established in solution, but the data related to the stability of the formed species, essentially depending on the concentration of the ligands, are elusive. For this reason, here, we report on a series of investigations where diverse approaches are combined together in order to characterize the behavior of As(III) species in aqueous solutions where simple chelating agents, such as thiolactic and thiomalic acids, are solvated. By synergistically exploiting potentiometric, calorimetric, and spectroscopic measurements along with ab initio molecular dynamics, the stability and the underlying formation mechanisms of specific species, along with the arsenic coordination modalities with the ligands, have macroscopically and microscopically been assessed. Furthermore, vibrational modes of the complexes formed by arsenic and simple thioacids have been assigned by means of Raman experiments.

The use of a vanadium species as a catalyst in photoinduced water oxidation.

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

Poly[(µ4-decanedio-ato)cobalt(II)].

In the title compound, [Co(C10H16O4)] n , the Co(II) atom is bonded in a slightly distorted tetra-hedral environment by four O atoms from the bridging sebacate dications, comprising two separate half-ligands which lie across crystallographic inversion centres. In the three-dimensional network coordination polymer, there are two different spatial extensions of Co(II) atoms, one with the Co(II) atoms lying parallel to (100) [Co⋯Co = 4.653 (1) Å], the other lying parallel to (010) [Co⋯Co = 4.764 (1) Å].

{2,6-Bis[(pyridin-2-yl)sulfanylmeth-yl]pyridine-κ(2) N,N'}(η(3)-prop-2-en-yl)palladium(II) hexa-fluoro-phosphate.

The title compound, [Pd(C3H5)(C17H15N3S2)]PF6, is built up by a [(η(3)-all-yl)Pd](2+) fragment coordinated by a 2,6-bis-[(pyridin-2-yl)sulfanylmeth-yl]pyridine ligand coordinated through the N atoms. One of the S atoms is at a close distance to the metal centeratom [3.2930 (8) Å]. The Pd(II) atom is tetra-coordinated in a strongly distorted square-planar environment mainly determined by the η(3)-allyl anion in which the central C atom is disordered over two equally occupied positions. The crystal packing is very compact and is characterized by a three-dimensional network of C-H⋯F interactions between the F atoms of each anion and several H atoms of the surrounding cationic complexes.

Tetra-kis(µ-4-chloro-benzoato-κ(2) O:O')bis-[(ethanol-κO)copper(II)](Cu-Cu).

In the centrosymmetric dinuclear title Cu(II) complex, [Cu2(C7H4ClO2)(C2H5OH)2], the Cu-Cu distance is 2.5905 (4) Å. The two metal atoms are bridged by four 4-chloro-benzoate ligands and each has an ethanol mol-ecule in the axial position of the overall octahedral coordination environment. The crystal packing features O-H⋯O hydrogen bonds.